Anode materials are used in electrochemical cells, and particularly in solid electrolyte fuel cells and in electrolytic cells having working temperatures ranging between 500 and 950° C. The materials used in the composition of such cells must meet several requirements in order to be employed as anodes. They must exhibit excellent catalytic properties for the electrochemical conversion of the gases on the electrodes, and ensure the transport of the electric current and the ionic current, while they must also have sufficient stability over many temperature cycles and over long periods of time.
A variety of material compositions, technologies, and designs are known from the literature for composition of high-temperature fuel cells at laboratory and pilot production scales. In the electrochemical cells mentioned above, the anode plays the role of electrochemically converting the combustible gas (hydrogen, methane, or hydrocarbons having longer chains) into carbon dioxide, water, and electrons that are used as electric energy. To this end, the oxygen ions delivered by the electrolyte material are reacted with the combustible gas on a catalyst surface, the gaseous products being removed via the existing porosity of the material and the electrons being removed via an electrically conductive phase. In order to ensure sufficient electron, ion, and gas transport, structures having porosities between 20 and 50% and materials having high electrical and ionic conductivity are employed.
Most anodes employed today therefore consist of a nickel/ion conductor composite, wherein typically zirconium oxide (YSZ or ScSZ), which is completely or partially stabilized with yttrium oxide or scandium oxide, or lanthanide-containing (for example, Y, Sm, Gd and the like) cerium oxides (abbreviated as CYO, CSO, or CGO) are used as the ion conductor.
As an anode, an Ni/YSZ composite typically having 40% by volume porosity, 24% by volume Ni, and 36% by volume YSZ has approximately the following properties:
Electrical conductivity (at 800° C.): 100-400 S/cm
(pure nickel: 23,000 S/cm)
Ionic conductivity (at 800° C.): approx. 0.001-0.006 S/cm
(pure YSZ with 8 mole % Y2O3: 0.056 S/cm)
Thermal coefficient of expansion: 12-13×10−6 K−1 
In these “cermets”, the nickel assumes both the function of the electrical conductor and that of the catalyst.
The configuration of the fuel cell can differ in that, either an anode is simply applied to a thick electrolyte substrate, or the anode itself is used as the substrate. Typically, an additional anode layer, also referred to as the functional anode layer, which has an optimized microstructure (see FIG. 1), is then used to improve the electrochemical activity. The electrolyte then has a thickness of only 5-50 μm, and as a result of the reduced thickness, the ohmic resistance of the fuel cell is also decreased, thereby allowing for advantageous use of such anode-supported fuel cells at lower temperatures of 600-800° C.
A significant disadvantage of anode-supported fuel cells, however, is the mechanical instability of the anode substrate if it is once again exposed to an oxidizing atmosphere during or after operation. The reoxidation of the metallic nickel into nickel oxide is associated with a significant increase in volume, resulting in cracks in the anode substrate as well as the thin electrolyte layer. Thus, ingress of air into the anode chamber must be excluded, in order for fuel cells comprising Ni/YSZ anodes to be used.
In the past, a series of proposals have been made regarding the development of reoxidation-stable anodes, but these are usually unsatisfactory for anode substrates. In addition to the physical properties mentioned above, which should be aspired to, the anode substrate must also meet a very narrowly defined mechanical boundary condition, in order to prevent cracking of the electrolyte. During a change in gas, from oxidizing to reducing conditions, or vice versa, an anode substrate should not change by more than 0.05% of the edge length thereof (corresponding to 50 μm for a cell measuring 100×100 mm2). Ideally, no measurable change in the size should occur.
This is difficult to achieve because, during a change in gas, from oxidizing to reducing conditions, or vice versa, the valence states of many transition metal cations change, bringing about a change in the crystal lattice parameters.
For example, substituted cerium dioxides are not completely stable in a combustible gas atmosphere, which is to say at oxygen partial pressures of 10−15>P(O2)>10−22. Some of the Ce4+ ions in the crystal lattice are reduced to Ce3+ ions, resulting in two property changes. First, the reduction reaction prompts the formation of free electrons, and thus higher electronic conductivity. Secondly, the reduction of the cerium ions is associated with a significant increase in volume. The ion radius of Ce4+ ions is 9.7 pm, that of Ce3+ ions is 11.43 pm [1], which is known from A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) 4709. A material such as Ce0.8Gd0.2O1.9 expands by 0.17% in an atmosphere of Ar/4% H2/3% H2O at 800° C. and is therefore not usually suitable as an anode substrate. This tendency to an increase in length or volume can also be observed with perovskites, such as LaCrO3, LaMnO3, or LaFeO3, wherein, in this case, there is a reduction reaction from (Cr, Mn, Fe)4+ to (Cr, Mn, Fe)3+.
In the literature, pure ceramic anodes are frequently described as being reoxidation-stable. However, in some cases they still do not meet the criterion of negligibly small change in length, and are not suitable for use as anode substrates. Additionally, pure ceramic anodes are often not sufficiently catalytically active, so that they do not appear suited as anodes at operating temperatures from 600 to 800° C. and therefore should only be used for electrolyte-supported fuel cells at operating temperatures of >800° C.
Several of the proposed solutions known from literature are set forth below.
U.S. patent application 2003/0165726 A1 describes the modification of an Ni/YSZ anode with the aim of improved redox stability, in that the structure is modified by small and large pores, so that no damage occurs to the structure during oxidation of the nickel. In order to produce the ceramic reticular system made of YSZ, Al2O3, TiO2, doped CeO2, MgO or spinels, these substances, together with a metal oxide in the form of a fine powder, are mixed with a pore-forming material and a liquid to form a paste. This paste is applied as a layer and sintered. The result is a two-part system having macropores and micropores, the first system comprising the electrode material and the second system comprising metals. This proposal, however, is little suited for achieving an equivalent anode substrate, because large pores have the effect that thin anode and electrolyte layers, and particularly the functional anode layer, cannot be applied as thick layers.
U.S. patent application 2004/0001994 A1 includes a description of anodes based on cerium-modified strontium titanate anodes (optionally containing La, Sc, or Y for Sr and optionally containing Ni, Co, Cu, Cr, or Fe for Ti). In addition to the use of these anodes in electrolyte-supported cells, the inventors also consider using the anode in anode-supported cells (paragraph [0066]). This, however, is not desirable, as cerium-containing materials form poorly conductive mixed phases with YSZ as the electrolyte, as disclosed in A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) p. 4709, and considerably impair the performance of fuel cells. The presence of cerium is, however, required for the use of the disclosed anode compositions, in order to ensure catalytic activity.
The anodes could be better used with a cerium dioxide-based electrolyte in order to avoid the poorly conductive mixed phases. However, this is only possible at operating temperatures of <600° C., since the cerium dioxide layer is otherwise partially reduced and tends to crack. The disclosed low cell performance at temperatures of <800° C., however, allows little promise for use in this variation.
Paragraph [0094] of A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) p. 4709, addresses the dimensional stability of such cerium-modified strontium titanate anodes in more detail and establishes that a change of up to 0.1% was acceptable for an SOFC anode. This may be true for electrolyte-supported cells, but this threshold value is too high for anode-supported cells.
It is apparent from US 2003/0165726 A1 that coarser powders result in considerably inferior cell performance due to higher polarization resistance (see FIG. 22 in the same). The polarization resistance values indicated in Table 1 of the same are achieved for different compositions after sintering the anodes at 1000° C. Since higher sintering temperatures are also always associated with a coarsening of the powder particles, it can be concluded that significantly inferior performance is to be expected if the anodes are sintered at 1350-1500° C., as is common for anode substrates. The use of cerium-modified strontium titanate anodes in anode-supported cells is therefore arguably possible with lower cerium content, but in all probability will not produce good power densities for the cells that are comparable to the state of the art.
In the U.S. patent application 2005/0250000 A1 the claims of the preceding patent application are extended to cerium dioxides having Nb, V, Sb, and Ta contents, however the disadvantages mentioned above remain in terms of use as an anode substrate.
The U.S. patent application 2004/0081893 A1 describes a material system, which comprises a component that expands during a change in the gas, from oxidizing to reducing conditions, and a component that contracts. As a result, the overall size of the components remains unchanged (dimensional stability). The expanding component comprises perovskites made of (La, Ca, Sr)(Cr, Fe)O3 and the contracting component comprises vanadium oxide, but is not specified in greater detail. This material combination demonstrates that dimensional stability can be achieved by the use of a multiple components. However, it is not suited for use as an anode or anode substrate because it is neither catalytically active, nor has any significant ion conductivity. Paragraphs [0109] et seq. also address the use of cerium dioxides, and the descriptions correspond to those of preceding patent applications, including the disadvantages there described.
A redox-stable Ni/YSZ anode is achieved in US 2004/0121222 A1 by impregnating a YSZ structure with Ni or NiO. In order to ensure electrical current conduction, the Ni content must be 10-30%. Such a Ni content on the surface of a ceramic structure results in high coverage of the ceramic surface. This not only brings about a decrease of the catalytically active centers (three-phase boundaries of Ni, YSZ and pores), but due to the close contact of the Ni particles, also results in an aggregation of Ni during operation of the cell, and thus a successive loss of electrical conductivity. Permeation with such Ni content appears to be disadvantageous with respect to the long-term stability of such anode substrates.